As is known, a radial tire comprises a tread, two inextensible beads, two sidewalls connecting the beads to the tread, and a belt placed circumferentially between the carcass reinforcement and the tread. This belt is made up of various rubber plies (or “layers”) which may or may not be reinforced by reinforcing elements (called “reinforcers”) such as cords or monofilaments, of the metal or textile type.
The belt generally consists of several superposed belt plies, sometimes called “working” plies or “crossed” plies, the generally metallic reinforcing cords of which are placed so as to be practically parallel to one another within a ply but crossed from one ply to another, that is to say they are inclined, whether symmetrically or not, to the median circumferential plane. These crossed plies are generally accompanied by various other auxiliary rubber plies or layers, which vary in width depending on the case and may or may not comprise metal reinforcers. Mention may in particular be made of what are called “protective” plies responsible for protecting the rest of the belt from external attack, from perforations, or else what are called “hoop” plies having metallic or non-metallic reinforcers oriented substantially in the circumferential direction, (so-called “zero-degree” plies), irrespective of whether they are radially outer or inner in relation to the crossed plies.
As is known, such a tire belt must meet various, often contradictory, requirements, in particular:                it must be as rigid as possible at low deformation, as it contributes substantially to stiffening the tire crown;        it must have as low a hysteresis as possible, in order, on the one hand, to minimize heating of the inner region of the crown during running and, on the other hand, to reduce the rolling resistance of the tire, this being synonymous with fuel economy; and;        finally, it must have a high endurance, in particular with respect to the phenomenon of separation, cracking of the ends of the crossed plies in the shoulder region of the tire, known as “cleavage”, which in particular requires metal cords that reinforce the belt plies to have a high compressive fatigue strength, when in a relatively corrosive atmosphere.        
The third requirement is particularly important in the case of tires for industrial vehicles, such as heavy goods vehicles or civil engineering machinery, which are designed in particular to be able to be retreaded one or more times when their treads reach a critical stage of wear after prolonged running or usage.
Furthermore, as is known, tire belts for industrial vehicles comprise one or more so-called “protective” crown plies or layers located beneath the tread and surmounting the working crown ply or plies, responsible for protecting the rest of the belt from external attack, tearing or other perforations.
These protective plies or layers must be sufficiently flexible and deformable so as, on the one hand, to best match the shape of the obstacle on which the belt bears during running and, on the other hand, to prevent the penetration of foreign bodies radially into the belt. Fulfilment of such requirements demands, as is known, the use in these protective layers of reinforcements or cords having a high elasticity and a high fracture energy.
To reinforce the protective crown plies, the hoop plies and the working crown plies for the tires of heavy goods vehicles, it is generally practice nowadays to use stranded cords, especially multistrand cords having a single layer (i.e. with no central core of one or more other strands) of K×(L+M) construction, the K elementary strands of which are assembled, wound simultaneously in a helix as a single layer, with an assembly pitch PK; each of the K two-layer elementary strands, of L+M construction, itself comprises a plurality of steel wires also wound together in a helix as two concentric layers (an inner layer of L wires and an outer layer of M wires).
Such stranded cords of K×(L+M) construction, especially of the high-elongation type, are well known and have been described in many patent documents, in particular for reinforcing the protective crown plies of tires for industrial vehicles such as heavy goods vehicles or civil engineering vehicles (see for example EP 1000074, U.S. Pat. No. 6,475,636, U.S. Pat. No. 7,458,200, WO 2004/003287 or US 2005/0183808, WO 2004/033789 or U.S. Pat. No. 7,089,726, or else RD (Research Disclosure) No. 33877, June 1992, 488-492).
As is well known by those skilled in the art, these multistrand cords must be impregnated as much as possible by the rubber in the tire belts that they reinforce, so that this rubber penetrates as much as possible into spaces between the wires constituting the strands. If this penetration is insufficient, empty channels then remain along the strands, and corrosive agents, for example water, capable of penetrating the tires, for example as a result of the tire belt being cut or otherwise attacked, travel along these channels through said belt. The presence of this moisture plays an important role, causing corrosion and accelerating the fatigue process (so-called “fatigue-corrosion” phenomena) compared to use in a dry atmosphere.
All these fatigue phenomena, generally grouped together under the generic term “fatigue-fretting corrosion”, are the cause of progressive degeneration of the mechanical properties of the cords and strands and may, under the most severe running conditions, affect the lifetime of the latter.
Moreover, it is known that good penetration of the cord by rubber makes it possible, because of the small volume of air trapped in the cord, to reduce the cure time of the tires (shortened “in-press time”).
However, the constituent elementary strands of these multistrand cords have, at least in certain cases, the drawback of not being able to be penetrated right to the core.
This is in particular the case for elementary strands of 3+M or 4+M construction, because of the presence of a channel or capillary at the centre of the three core wires, which remains empty after external impregnation with rubber and therefore propitious, through a kind of “wicking” effect, to the propagation of corrosive media such as water. This drawback of the strands of 3+N construction is well known; it has been explained for example in the patent applications WO 01/00922, WO 01/49926, WO 2005/071157 and WO 2006/013077.
To solve this problem of penetrability right to the core of cords of 3+M construction, patent application US 2002/160213 has certainly proposed producing strands of the type rubberized in situ. The process proposed here consists in sheathing, individually (i.e. in isolation, “wire to wire”) with rubber in the uncured state, upstream of the point of assembly (or twisting point) of the three wires, just one or preferably each of the three wires in order to obtain a rubber-sheathed inner layer before the M wires of the outer layer are subsequently put in place by being corded around the inner layer thus sheathed.
The above application provides no information relating to the construction of the 3+M strands, in particular neither information about the assembly pitches nor information about the amounts of filling rubber to be used. Furthermore, the proposed process poses many problems.
Firstly, the sheathing of one single wire in three (as illustrated for example in FIGS. 11 and 12 of this patent application US 2002/160213) does not guarantee sufficient filling of the final strand with the rubber and therefore prevents satisfactory corrosion resistance being obtained. Secondly, the wire-to-wire sheathing of each of the three wires (as illustrated for example in FIGS. 2 and 5 of that document), although effectively filling the strand, leads to the use of an excessively large amount of rubber. The overspill of rubber at the periphery of the final strand then becomes unacceptable under industrial cabling and rubber-coating conditions.
Because of the very high adhesion of rubber in the green (i.e. uncrosslinked) state, the strand thus rubberized becomes unusable because of the undesirable adhesion to the manufacturing tools or between the strand turns during winding of the latter onto a take-up reel, without even mentioning the final impossibility of correctly calendering the cord. It will be recalled here that calendering consists in converting the cord, by incorporation between two layers of rubber in the green state, into a rubberized metal fabric serving as semi-finished product for any subsequent manufacture, for example to produce a tire.
Another problem posed by the insulated sheathing of each of the three wires is the large amount of space required by using three extrusion heads. Because of such a space requirement, the manufacture of cords having cylindrical layers (i.e. with different pitches p1 and p2 from one layer to another, or with identical pitches p1 and p2 but with different twisting directions from one layer to the other) must necessarily be carried out in two batch operations: (i) individual sheathing of the wires followed by cabling and winding of the inner layer in a first step; and (ii) cabling of the outer layer around the inner layer in a second step. Again because of the high adhesion of rubber in the green state, the winding and intermediate storage of the inner layer require the use of spacers and many separators during winding onto an intermediate reel, so as to avoid undesirable adhesion between the coiled layers or between the turns of a given layer.
All the above constraints are greatly prejudicial from the industrial standpoint and in conflict with the aim of producing high manufacturing rates.